bgwriter tunables vs pg_stat_bgwriter

Since getting on 8.4 I've been monitoring things fairly closely.
I whipped up a quick script to monitor pg_stat_bgwriter and save
deltas every minute so I can ensure my bgwriter is beating out the
backends for writes (as it is supposed to do).

Now, the odd thing I'm running into is this:

bgwriter_delay is 100ms (ie 10 times a second, give or take)
bgwriter_lru_maxpages is 500 (~5000 pages / second)
bgwriter_lru_multiplier is 4

Now, assuming I understand these values right the following is what
should typically happen:

while(true)
{
if buffers_written > bgwriter_lru_maxpages
or buffers_written > anticipated_pages_needed *
bgwriter_lru_multiplier
{
sleep(bgwriter_delay ms)
continue;
}
...
}

so I should not be able to have more than ~5000 bgwriter_clean pages
per minute. (this assumes writing takes 0ms, which of course is
inaccurate)

However, I see this in my stats (they are deltas), and I'm reasonably
sure it is not a bug in the code:

(timestamp, buffers clean, buffers_checkpoint, buffers backend)
2010-02-17 08:23:51.184018 | 1 | 1686
| 5
2010-02-17 08:22:51.170863 | 15289 | 12676
| 207
2010-02-17 08:21:51.155793 | 38467 | 8993
| 4277
2010-02-17 08:20:51.139199 | 35582 | 0
| 9437
2010-02-17 08:19:51.125025 | 8 | 0
| 3
2010-02-17 08:18:51.111184 | 1140 | 1464
| 6
2010-02-17 08:17:51.098422 | 0 | 1682
| 228
2010-02-17 08:16:51.082804 | 50 | 0
| 6
2010-02-17 08:15:51.067886 | 789 | 0
| 1

perhaps some stats buffering occurring or something or some general
misunderstanding of some of these tunables?

--
Jeff Trout <jeff(at)jefftrout(dot)com>
http://www.stuarthamm.net/
http://www.dellsmartexitin.com/